engine wouldn't start

[TheJ]

So yesterday I went to start my mustang and it would fire for like a second then die. Today same thing, the only thing I did since the last time it ran was remove the gauge cluster. Come to find out (I think) the tach grounds the ign coil. Hooked the tach back up and it started and ran like a dream.

Is this how it's supposed to work or did I wire the coil wrong?

Thanks for looking.

 

[marks73]

Take a look at this

http://www.mre-books.com/shopmanuals/download/demo10023.pdf

 

[TheJ]

Thanks but there's only two pages of wire diagrams. I'll check my factory shop manual.

 

[OLE PONY]

Not sure if it was the tach, but you might try unhooking the Tach again to see if the engine will start! Could have been a fluke.

 

[TheRktmn]

The factory tach is wired as part of the ignition circuit - in a nutshell: 12V+ from ignition switch through tachometer, through resistor wire to ignition (+) side of coil. If the tach fails or is unplugged there is no power to the coil so the car will not run. It will fire when the key is in start because the bypass wire from the solenoid is supplying 12V to the coil during start.

 

[OLE PONY]

Good info, so not a fluke! :udaman:

 

[72HCODE]

Yup need a tach or bypass the tach by jumping the plug to the tach with a paperclip.

 

[TheJ]

Not sure if it was the tach, but you might try unhooking the Tach again to see if the engine will start! Could have been a fluke.

When I had the tach hooked up it ran fine, I unplugged the tach and the motor shut off.

After looking at the wiring diagram I found out that there is supposed to be another wire spliced before the tach that goes to the coil and gives it a constant 12v when the igniton switch is in the on position. I just got in from testing my theory and it works great with a jumper and I found the wire so I just need to extend it to make it work.

Thanks for the info guys.

I highlighted the wire that's not connected with blue

 

[TheRktmn]

Jayson,

By using that wire you are bypassing the resistor wire and delivering a full 12V to the coil and (if you have them) points. This will shorten the life of both significantly.

As 72HCODE mentioned - if your tach is bad, unplug it and add a jumper to complete the circuit across the tach wire plug. This will keep the ignition circuit working as designed.

 

[TheJ]

Jayson,

By using that wire you are bypassing the resistor wire and delivering a full 12V to the coil and (if you have them) points. This will shorten the life of both significantly.

As 72HCODE mentioned - if your tach is bad, unplug it and add a jumper to complete the circuit across the tach wire plug. This will keep the ignition circuit working as designed.

I see your concern but why does the factory wire diagram show a switched 12v wire going from the starter relay/solenoid to the coil.

 

[TheRktmn]

The wire from the I post of the solenoid gives a full 12V to the coil when the solenoid is engaged (key in start position). It connects to the main ignition "run" wire that comes from the ignition through the tach (if equipped) and the resistor wire - after the resistor wire. The start wire will show voltage with the engine running, but it is back feeding from the run wire.

If you add a 12V source to that wire all the time you have hot wired the ignition. If you supply a keyed or "switched" 12V to that wire you have bypassed the tach and the resistor wire.

 

[TheJ]

Let me see if I can clear this up a bit. Some how my tach signal is connected directly to the coil right now and it is supposed to be (from what I can see by looking at the diagram) split at C-216 with one wire going to the tach and the other going to the keyed side of the starter solenoid. Those two wires become one at C-216 and go to the coil.

Is this correct or is the diagram I have incorrect?

Thanks again.

 

[TheRktmn]

Yes, the start wire from solenoid I post joins the run wire from the tach on the firewall side of the three pin connector coming through the firewall. On the firewall side it has OIL, TEMP, START + RUN (4 wires). On the engine side it has OIL, TEMP, COIL+.

 

[TheJ]

So my coil is wired wrong. Because, it may just be me, if for some odd reason the tach wire were to break or short I would think that ford didn't wire it so that the car would not run in such a case.

That's just my logic.

 

[TheRktmn]

Yep, you would think that, wouldn't you? It's a pretty common problem - lots of posts about it on Mustang and Cougar forums. Another item that will cause the same problem - mostly seen in non-tach cars is that 'pink' resistor wire. It gets brittle with age & breaks. When that happens the car will fire but not run.

 

[TheJ]

So the car is acting as it should and I shouldn't worry about it.

My question is why does the diagram say otherwise and why did I find the wire ,I think, that is supposed to go to the coil?

 

[Tnfastbk]

So the car is acting as it should and I shouldn't worry about it.

My question is why does the diagram say otherwise and why did I find the wire ,I think, that is supposed to go to the coil?

Maybe the last owner thought the same, you never know

 

[shellbuyer]

so then, I may have not read and understood, but if the tach makes a difference in whether the car starts or not, what if as in my 351 H code has no tach? Should I only have two wires going to coil? One has 12 volts that comes from engine harness and the other (neg) comes from dizzy?